A photosensitive printing plate in which photosensitive layer and a oxygen-barrier layer are sequentially laminated on a substrate has been widely used in the offset printing heretofore. With the aid of the recent technological advancement of electronic devices, the speed of manufacturing a printing master plate has been greatly, increased, since a process from writing a manuscript, input of image data, editing, proofreading to manufacturing of a printing master plate has been placed as a whole under control of a computer system even in the field of printing.
Exposure to a visible laser light is suitable for a high speed plate manufacturing, because argon laser of a wavelength of 488 nm is especially preferred due to the maskless exposure. As a photosensitive printing plate suitable for such a laser exposure, one for instance, as typically disclosed in the specification of U.S. Pat. No. 3,106,466 and the like is proposed. However, the aforementioned photosensitive printing plate does not have a wide range of wavelengths in photosensitivity enough to have the feature of the laser exposure fully exerted. For the reason, a photosensitive printing plate was proposed in Japanese Patent Application No. SHO 60-239,736(hereinafter abbreviated as JP-A-60-239,736 and the abbreviation is generally applied to the other) in which a sensitizer comprising an s-triazine compound was contained and thereby the photosensitive range of wavelengths was expanded to a range from near ultraviolet to visible light.
The photosensitive printing plate thus proposed in JP-A-60-239,736 is high in sensitivity, but has the faults that the degree of distinctiveness of pattern elements (hereafter simply referred to as degree of pattern distinctiveness or pattern distinctiveness) after exposure and development is poor, part of unexposed resin is left behind, and much time is required for inspecting chipping or peeling off of pattern edges, which cause the working efficiency to be lowered.
In order to improve the degree of pattern distinctiveness, a method was proposed in JP-A-5-72739 in which transparent inorganic particles were added in the oxygen-barrier layer of a photosensitive printing plate. A sensitivity was degraded because of the additive being inorganic particles and thereby the sensitivity was not able to reach a necessary level for a high speed plate manufacturing.
In light of such a present state, the inventors have conducted intensive investigations on the above-mentioned problems. It has been found that a photosensitive resin composition comprising the following components of an optically-radical-generating agent that generates a radical in the mass of the resin composition on exposure to light of a specific wavelength, an acid-generating agent that generates an acid on exposure to light of a shorter wavelength than the above-mentioned specific wavelength and a chemical compound developing a color with the action of the acid, is capable of being sensitive on exposure to visible laser light, and that a photosensitive printing plate with the resin composition layer laminated thereon has good characteristics such as a high sensitivity, a high resolution power, easy handling, and good storage stability. Besides, a method of manufacturing a printing master plate has been also found. The present invention has been completed on the basis of the above findings.